DE3877444T2 - METHOD FOR PRODUCING A CERAMIC / METAL HEAT STORAGE MATERIAL AND CORRESPONDING PRODUCT. - Google Patents

Abstract

A heat storage medium comprising a body of parent metal and an intrinsically cohesive ceramic layer formed integrally with the metal body and encapsulating said metal body is produced by the directed oxidation of a body of parent metal outwardly from the surface of said body to form integrally with the body of parent metal a layer of oxidation reaction product which encapsulates unreacted parent metal and forms a cavity resulting from the depletion of parent metal.

Description

The present invention relates to a method for producing direct contact heat storage media consisting of a metal body encapsulated by a one-piece ceramic container, and the product of the method. More particularly, the invention relates to a method for producing a heat storage medium by the directed oxidation of a mass of base metal to form a ceramic coating integrally formed with and encapsulating unreacted metal which undergoes a melt-solidification transformation when used as a heat storage medium.

Background of the invention, description of the state of the art and patent applications of the same applicant

Metals are known to have high thermal conductivity compared to various other materials and are being investigated for their suitability as heat storage media. In such applications, the metal as a storage medium undergoes successive cycles of melting after heating and solidification after cooling and is therefore generally referred to as a phase change material. Some metals (and alloys) exhibit relatively high latent heats of transformation and also offer an important advantage for use in heat exchange and storage in that the ratio of heat exchange area to storage volume for a given cycle time can be much smaller than for materials that exhibit poorer thermal conductivity. Furthermore, a latent heat of melting is absorbed at the melting point of any material. However, metals are generally not usable at temperatures where changes in latent heat could be advantageously used in heat storage media because they lose their shape and strength when melted.

In such a case, an advantageous container for a heat storage medium made of metal would allow heat exchange between the exterior of the container and the metal and maintain its mechanical properties despite the phase transitions (melting and solidification) of the metal contained therein. Also, an encapsulated phase change material would allow its direct contact with an energy transporting fluid. A container made of ceramic that is able to transfer heat to the metal but is still structurally stable enough to contain the metal used at the temperatures of use would meet these criteria.

US Patent 4,146,057 (issued March 27, 1979 to J. Friedman et al.) discloses an energy storage system for buffering interruptions and/or asynchrony between an energy supplier and an energy consumer. The energy storage system includes a buffer portion consisting of a ceramic container filled with aluminum and coupled to a potassium loop and a power and energy loop. Apart from Despite the generalized statement that a ceramic container is useful for housing aluminum in a heat storage system, there is no disclosure or suggestion as to how the ceramic body is manufactured.

U.S. Patent 2,823,151 (issued February 11, 1958 to Yntema et al.) discloses the formation of a skin of an alloy or intermetallic compound on a metal substrate, particularly a molybdenum metal substrate, to make the substrate resistant to oxidation at high temperatures. The skin is described as a molybdenum-silicon-boron alloy or intermetallic compound and is formed on the molybdenum metal substrate by reacting the parent molybdenum with silicon and boron, or by coating a non-molybdenum metal substrate with molybdenum and then reacting the molybdenum with silicon. However, this patent is directed exclusively to a method of coating and in no way suggests a heat storage medium since the molybdenum is not melted nor undergoes a melt-solidification transformation. Furthermore, Yntema et al. do not disclose oxidation of the metal base or substrate to produce an encapsulating metallic container capable of containing the molybdenum metal in the molten state.

British Patent Application 2,159,542 (filed March 13, 1985 to Zielinger et al.) relates to a process for producing isotropic protective oxide layers on metal surfaces, the rate of growth of the layer being controlled by varying the oxygen pressure in the growth environment. However, Zielinger et al. do not disclose or suggest growing a ceramic layer of appreciable stability containing the coated metal in the molten state, nor do they suggest forming a heat storage medium.

U.S. Patent 4,657,067 to Rapp et al. discloses a thermal storage material that utilizes the heat of fusion of eutectic alloys by forming the outer shell to have a higher melting point than the eutectic core. The material is formed by melting a phase change alloy and then allowing the melt to cool slowly so that the high melting material, specifically pure silicon or a silicide contained in the overall composition, solidifies first and encapsulates the lower melting material of the inner eutectic core.

A novel and useful method for producing self-supporting ceramic bodies by the directed oxidation of a mass of precursor metal (base metal) is disclosed in our following patent applications. The directed oxidation process is suitable for the process of producing a heat storage medium comprising a self-encapsulating metal.

Accordingly, the patent application EP-A-1 55831 of the same applicant describes in general terms the process for producing ceramic materials by the directed oxidation of a molten base metal. In this process, an oxidation reaction product first forms on the surface of the body of molten base metal exposed to an oxidizing agent and then develops outward from this surface, when molten metal is transported through the oxidation reaction product and into contact with the oxidant at the interface between the oxidant and previously formed oxidation reaction product where it reacts to form an increasingly thick layer of oxidation reaction product. The process can be improved by the use of dopants alloyed into the parent metal, as in the case of an aluminum parent metal oxidized in air. This process has been improved by the use of dopants coated on the outer surface of the parent metal, as disclosed in EP-A-169067 of the same applicant. In this context, oxidation is understood in the broadest sense of the word and is intended to mean that one or more metals donate electrons to or share electrons with another element or combination of elements to form a compound. Accordingly, the term "oxidant" refers to an electron acceptor or an electron sharing substance.

In the process described in the applicant's EP-A-193292, ceramic composite articles are made by growing a polycrystalline ceramic product into a bed of filler material adjacent to a body of molten parent metal. The molten metal reacts with a gaseous oxidant, such as oxygen, to form a ceramic oxidation reaction product which permeates the filler. The resulting oxidation reaction product, e.g. alumina, can grow into and through the mass of filler as molten metal is continually drawn through the oxidation reaction product and reacts with the oxidant. The filler particles become embedded in the polycrystalline ceramic product, which is a composite oxidation reaction product. Our patent applications do not disclose that the directed oxidation process is adapted for the formation of a ceramic container around a metal body for forming a heat storage medium. However, the present invention provides a method for using the directed growth process for developing a ceramic container around a metal body for forming a heat storage medium.

The applicant's patent application EP-A-245192 (not prepublished) discloses particularly effective processes in which the filler is formed as a preform having a shape corresponding to the desired external dimensions of the final composite article. The preform is prepared by conventional methods such that its shape has sufficient integrity and green strength and should be permeable for the transport of the oxidation reaction product. A mixture of filler materials and particle sizes may also be used.

Barrier materials may be used to inhibit or arrest the growth of the oxidation reaction product at selected boundaries to determine the shape or external dimensions of the ceramic structure. This invention is disclosed in the applicant's patent application EP-A-2451 93 (not prepublished).

Patent applications EP-A-234704 and EP-A-259239 (both not prepublished) disclose processes for producing ceramic bodies containing a cavity from a size and thickness which have been difficult or impossible to duplicate with previously known technologies. Briefly summarized, the inventions described therein involve embedding a shaped parent metal precursor in a conformable filler and infiltrating the filler with a ceramic matrix obtained by oxidizing the parent metal to form a polycrystalline product of the oxidation reaction of said parent metal with an oxidizing agent and, if desired, one or more metallic constituents. More particularly, in the practice of the invention, the parent metal is given a mold to provide a pattern and then placed in or surrounded by a conformable filler which inversely repeats the external dimensions of the shaped parent metal. In this process (1) the filler is permeable to the oxidizing agent when required, as in the case where the oxidizing agent is a vapor phase oxidizing agent, and in any event is permeable to infiltration by the developing oxidation reaction product; (2) the filler has sufficient conformability over the temperature range of the heating phase to accommodate the difference in thermal expansion of the filler and the base metal plus any volume change of the metal at the melting point; and (3) the filler is inherently self-binding, if necessary, at least in a support zone surrounding the pattern, whereby said filler has sufficient dimensional stability to maintain the inversely repeated external dimensions in the mount upon migration of the base metal as described below. The surrounded or embedded molded base metal is heated to a temperature range above its melting point but below the melting point of the oxidation reaction product to form molten base metal. The molten base metal is reacted with the oxidizing agent in this temperature interval or range to form the oxidation reaction product. At least a portion of the oxidation reaction product is maintained in contact with and between the body of molten parent metal and the oxidant in said temperature range, whereby molten metal is progressively drawn from the body of molten metal through the oxidation reaction product, simultaneously forming the void as the oxidation reaction product continues to form in the filler bed at the interface between the oxidant and previously formed oxidation reaction product. This reaction is continued in said temperature range for a time sufficient to at least partially embed the filler in the oxidation reaction product by growth of the latter to form the composite body containing the aforementioned void. Finally, the resulting self-supporting composite body is separated from excess filler, if any.

The entire disclosure of all prior patents issued to the same owner are expressly incorporated herein by reference.

Summary of the findings

According to the present invention, there is provided a method of making a heat storage medium comprising a metal core or body and an inherently coherent ceramic container. The ceramic body is formed integrally with and encapsulates the metal substrate by reacting a portion of the body of molten parent metal with an oxidizing agent, preferably a vapor phase oxidizing agent. Thus, the ceramic container is comprised of an oxidation reaction product of a molten parent metal and an oxidizing agent, and the heat storage medium is comprised of a remaining body of unoxidized parent metal which has not been reacted to form the container.

The body of parent metal is heated in the presence of the oxidizing agent to a temperature above the melting point of the parent metal but below the melting point of the oxidation reaction product, thereby forming a body of molten parent metal. At this temperature, molten parent metal on the outside of the surface of the body of parent metal is reacted to form a layer of oxidation reaction product, thereby beginning to encapsulate the unreacted body of molten parent metal. Molten parent metal is transported through the encapsulating oxidation reaction product and into contact with the oxidizing agent at the interface between the oxidizing agent and previously formed oxidation reaction product, thereby forming an increasingly thick coating or container of oxidation reaction product that develops outward from the surface of the body of molten parent metal and consumes some of the underlying molten parent metal.

The molten parent metal is transported through the layer of reaction product and into contact with the oxidant and reacted therewith for a time sufficient to develop an encapsulating layer of oxidation reaction product as a ceramic matrix container having sufficient wall thickness to enclose the remaining body of parent metal consisting of unreacted or unoxidized parent metal and to withstand stresses during use when the metal body encapsulated by the shell may be melted. This thickness of the shell depends on such factors as the type and composition of the parent metal, the dimensions of the parent metal body, and the parameters or conditions of ultimate use. Thus, the resulting product is a body of parent metal comprising unreacted or unoxidized parent metal enclosed by a substantially coherent ceramic container consisting of the product of the oxidation of the molten parent metal with the oxidant. The amount and volume of unreacted or unoxidized base metal enclosed by the ceramic container made according to the present invention is less than the amount or volume of base metal making up the base metal body which is the precursor to the process. Thus, a space or void develops in the ceramic container as a result of consumption of a portion of the base metal reacted to form the ceramic container. This void or cavity can accommodate the expansion of the metal body during its use as a storage medium, which would otherwise result in cracking or failure of the ceramic body due to the volume change upon melting of the encapsulated body or the mismatch between the thermal expansion of the ceramic container and the underlying metal body.

In a preferred embodiment, the container consists of a ceramic matrix embedding a suitable filler, preferably a ceramic filler. A mass of filler material is placed adjacent to the surface of a parent metal, preferably by applying the filler as a coating to the parent metal body. Any suitable filler material may be used, such as metal oxides, borides, carbides or nitrides (e.g., particles, fibers or whiskers of alumina or silicon carbide). A coating of filler material may be applied to the surface of the parent metal body using a suitable carrier, such as an organic binder that burns or volatilizes during the process to provide sufficient internal stability to the coating. The parent metal body is then heated and the molten parent metal reacts with the oxidizing agent to form an encapsulating layer of oxidation reaction product, as described above. The developing oxidation reaction product infiltrates the filler material, and the oxidation reaction is continued for a time sufficient for the filler material to become embedded or infiltrated by the oxidation reaction product. The resulting ceramic container of this embodiment consists of a composite body of the ceramic oxidation reaction product and the filler material.

In yet another embodiment, the filler material coating of the present invention may comprise a material that reacts with certain molten parent metals to form a ceramic support zone so that the molten parent metal is confined or supported during formation of the ceramic container, such as silica for a parent metal system of aluminum and air as an oxidant. The reaction of the molten parent metal with the oxidant may be preceded by or accompanied by a reaction of the parent metal with the coating material.

In another embodiment, especially when a filler material is used, a suitable material as described in the above-discussed EP-A-245193 of the same applicant is placed adjacent to the coating of filler material and thus is located opposite the outer surface of the parent metal. Such a barrier material, which is preferably gas permeable, inhibits or substantially arrests the development of the oxidation reaction product on the barrier material, thereby controlling the wall thickness of the ceramic container. This barrier material may, for example, consist of a mixture or slurry of gypsum and water, or of a bed of particles or fibers such as wollastonite (a mineral calcium silicate).

The following terms, as used in this specification and the accompanying claims, are defined as follows:

"Ceramic" is not intended to be strictly limited to a ceramic body in the classical sense, i.e. in the sense of being composed entirely of non-metallic and inorganic materials, but rather refers to a material that is predominantly ceramic either in terms of its composition or in its predominant properties, although the material may contain minor or major amounts of one or more metallic constituents and/or porosity (continuous and isolated) derived from the parent metal or derived from the oxidizer or a dopant by reduction, most typically within a range of about 1-40 volume percent, but may be higher.

"Oxidation reaction product" generally means one or more metals in an oxidized state in which a metal has donated or shares electrons with another element, compound, or combination thereof. Accordingly, an "oxidation reaction product" under this definition includes the product of the reaction of one or more metals with an oxidizing agent, such as those described in this application.

"Oxidant" means one or more suitable electron acceptors or electron sharing substances and may be a solid, a liquid or a gas (vapor) or a combination of these (e.g. a solid and a gas) under the process conditions.

"Base metal" shall refer to relatively pure metals, to commercially available metals containing impurities and/or alloying components, and to alloys and intermetallic compounds of the metals. When a specific metal is mentioned, that specified metal should be read in light of this definition unless the context indicates otherwise.

Short description of the drawings

FIGURE 1 is a cross-sectional view through the center of a substantially cylindrical heat storage medium made in accordance with the present invention.

FIGURE 2 is a top view photograph of the heat storage medium made in accordance with the present invention.

Detailed description and preferred embodiments

In the practice of the present invention, a base metal, which may be doped (as described in more detail below), is formed into a suitable shape or body, e.g., a sphere, disk, rod, or the like. The surface of the shaped body of base metal is exposed to, made accessible to, or in contact with an oxidizing agent or oxidizing environment, preferably a vapor phase oxidizing agent. The surface of the body, as used herein, refers to the outer surface or surfaces, or a portion thereof, that is exposed to the oxidant. Thus, the surface of the body may include one or more surfaces, sides, faces, holes, openings, projections, flanges, or the like. Typically, the metal body is placed on a refractory support, such as a bed of refractory particles that is permeable to a vapor phase oxidant when required, and that is relatively inert under the process conditions in that the support is not wetted by the molten metal. If the base metal body is supported on an inert bed, then the metal body may lose its shape during heating, but this loss of shape is usually minor and does not affect the ultimate use of the product. If desired, the metal body may be in contact with a solid or liquid oxidant, or it may be coated with a filler which is permeable to the growth of the oxidation reaction product, as explained in more detail below, thereby avoiding loss of shape. The structure is contained in a suitable refractory crucible. In the embodiment in which the ceramic container embeds a filler material (as described in more detail below), the metal body may be directly submerged in a bed of a suitable filler material, such as particles, fibers or whiskers, contained in a refractory crucible, or it may first have a coating of filler material applied to its metal surface and then stored in a crucible. Such fillers are typically ceramic, e.g. alumina, silica or silicon carbide, and the directed growth of oxidation extends to a desired or predetermined depth, as described in more detail below.

The resulting assembly of the base metal body, a support device, or a bed of refractory particles, typically arranged or contained in a suitable refractory crucible or container, is heated in an oxidizing environment to a temperature above the melting point of the base metal but below the melting point of the oxidation reaction product. However, the operable or preferred temperature range need not extend over the entire temperature range between the melting points of the base metal and the oxidation reaction product. Accordingly, at or within this temperature range, the base metal melts to form molten base metal, and upon contact with the oxidant, the molten metal reacts to form a layer of oxidation reaction product, thereby encapsulating or entrapping the unreacted molten base metal. With continued exposure to the oxidizing environment, molten base metal is transported into and through the previously formed encapsulating layer of oxidation reaction product toward the oxidant. The transported molten metal contacts the oxidant at the interface between the oxidant, typically the atmosphere, and previously formed oxidation reaction product, whereby an increasingly thick encapsulating layer of oxidation reaction product separates from the surface of the ceramic body to the outside and at the same time a certain amount of the molten metal is consumed. As the ceramic container develops by consuming the base metal inside the container and reacting this metal with the oxidizing agent to the outside, a void or shrinkage cavity forms in the container.

Typically, the ceramic container for a heat storage medium made in accordance with the invention is thin relative to the thickness or dimensions of the unreacted parent metal remaining in the container. Thus, the cavity inevitably formed in the present process can accommodate the expansion of the metal substrate upon heating and melting in use, which would otherwise cause failure of the ceramic container due to the mismatch of the thermal expansion of the metal substrate and the ceramic container and the volume change as the metal melts. The reaction is continued for a time sufficient to develop an appropriate wall thickness for the encapsulating ceramic layer, coating or container, thereby forming the container integral with the underlying unreacted parent metal. It will be understood, however, that although the ceramic container is formed integrally with the underlying unreacted base metal and a space or void volume develops, the position of this void and/or contained metal may move, shift or otherwise change or redistribute during use of the heat storage medium at temperatures above the melting point or liquidus temperature of the alloy of the metal body. The appropriate wall thickness will depend on factors related to the particular embodiment, such as the type and composition of the base metal, the dimensions and external shape of the base metal body, the parameters of the application and the mechanical stresses to which the heat storage medium is subjected during use. For example, a ceramic container approximately 1 to 2.5 mm (0.04 to 0.1 in) thick, developed from an approximately 2.54 cm (1 in) spherical body of aluminum alloy as the base metal, has been found to be sufficient to adequately contain the non-oxidized base metal substrate under moderate external stress at application temperatures above the liquidus temperature of the aluminum alloy.

The ceramic container of the present invention consists of the product of the oxidation reaction of the parent metal with the oxidizing agent. It should be understood that the oxidation reaction product forming the ceramic container may contain unreacted parent metal and/or porosity resulting from partial or nearly complete replacement of the metal, but the volume percent of parent metal and/or voids in the ceramic container depends in large part on such conditions as temperature, time, and the nature of the parent metal. The oxidation reaction product phase is in the form of crystallites which are at least partially interconnected, preferably in three dimensions. Thus, the ceramic container has many of the desirable properties of a classical ceramic (i.e., hardness, heat resistance, wear resistance, etc.) while deriving additional benefits (in cases where a significant metal phase is present) from the presence of the dispersed or transported unreacted metal phase, in particular higher fracture toughness and strength, and, more importantly in this context, higher thermal conductivity across the wall thickness of the ceramic container, thereby causing a more efficient energy transfer from the energy source outside the ceramic container to the metal substrate for storage.

In a preferred embodiment of the invention, a coating material of a suitable filler is applied to the surface of the base metal body. The coating material may consist of an inert filler material such as particles of aluminum oxide, aluminum nitride or silicon carbide, whiskers, fibers or the like. The filler material is applied to the outer surface of the base metal in any suitable manner and conforms to the outer dimensions of the metal body. For example, the filler material may be mixed with an organic binder such as polyvinyl alcohol or methyl cellulose to provide sufficient wet strength during molding and is eliminated by evaporation or volatilization at process temperatures. During heating and throughout the temperature interval, the coating of filler material should be able to accommodate the difference in thermal expansion of the filler and the base metal plus a volume change of the metal at the melting point. When the molten parent metal reacts with the oxidant, oxidation reaction product develops and infiltrates the bed or coating of filler material. Thus, the resulting ceramic container consists of a composite body with a ceramic matrix of oxidation reaction product embedding the components of the coating material. If desired, the coating material can consist of a material that at least partially reacts with the molten parent metal. For example, in the case of an aluminum parent metal that is oxidized in air, with alumina as the desired reaction product, silicon oxide or silicon compounds or boron or boron compounds can be used as the filler material. These compounds react, at least partially, with the molten aluminum parent metal. In such a case, the oxidation reaction of the parent metal with the oxidant can be preceded or accompanied by the reaction of the parent metal with the filler material. The coating of filler materials can also consist of mixtures of reactive and inert materials, such as those present in inorganic clays. It is thus possible to tailor the composition or properties of the coating.

In embodiments of the present invention where a coating of filler material is used and applied to the surface of the base metal, a barrier material as disclosed in the above-discussed patent application EP-A-245193 of the same applicant may be disposed adjacent to the coating material and thus opposite the surface of the body of base metal. As disclosed in the above-discussed patent application of the same applicant, the growth of the oxidation reaction product is substantially arrested by the barrier means so that the ceramic matrix is contained within the coating of filler material. As disclosed in the above-mentioned EP-A-2451 93, Suitable barrier agents may be any material, compound, element, composition or the like which maintains some integrity under the process conditions of this invention, is not excessively volatile and is preferably permeable to the vapor phase oxidant while at the same time being capable of locally inhibiting, poisoning, stopping, affecting, preventing or the like the continued growth of the oxidation reaction product. Suitable barriers for use with an aluminum parent metal and oxygen-containing gas type oxidants include calcium sulfate (gypsum), calcium silicate such as wollastonite, Portland cement and combinations thereof. Furthermore, if a barrier material is used, a suitable heat resistant particulate material may also be included to reduce possible shrinkage or cracking which may otherwise occur during the process upon heating and which would affect the morphology of the ceramic container. As discussed above, many of these barrier materials are inherently self-supporting when allowed to set or hydrolyze.

Although the invention is described herein with particular reference to aluminum as the preferred parent metal, other suitable parent metals that meet the criteria of the present invention and are useful heat storage media may also be used, such as silicon, titanium, zirconium, hafnium and tin. A heat storage medium made in accordance with the invention is shown in FIGURE 1. The parent metal is preferably a high entropy eutectic alloy, such as an aluminum-silicon alloy having a eutectic of 580°C at 12.5 weight percent silicon. Hypereutectic alloys and ternary or multicomponent alloys may also be useful for optimizing heat storage. The heat storage medium includes the ceramic container 4 which encapsulates the body of parent metal 6, which consists of the remaining untransported and unreacted parent metal. A space or void 8 resulting from the consumption of the base metal body has a volume sufficient to accommodate the expansion of the metal during use.

Certain base metals useful as heat storage media meet the criteria required for the directed oxidation reaction without special additions or modifications under certain conditions of temperature and oxidizing environment. For example, aluminum alloy 4032, which contains approximately 12 weight percent silicon and 1 weight percent magnesium, may be particularly suitable.

As stated above, the base metal of the present invention may be a relatively pure metal, e.g. aluminium, but preferably it is an alloy containing a significant amount of silicon and/or carbon, provided that the base metal is compatible with the oxidation reaction process. Therefore, the particular base metal may be selected to obtain the desired heat storage properties in a resulting heat storage medium or transfer medium. By varying the alloy composition of the base metal, the characteristics of the phase transitions of the metal body during use as the respective heat storage medium, thereby influencing the heat storage properties of the medium. The type of metal body of the present invention is therefore not limited to the parent metal, that is, the metal which reacts with the oxidizing agent to form the oxidation reaction product, e.g. aluminum in air to form aluminum oxide. For example, aluminum-silicon alloys generally have high heat storage densities because aluminum and especially silicon have high entropies of fusion. According to the present invention, a parent metal of an aluminum alloy having a silicon content of up to about 30 weight percent can be oxidized to form a container of an oxidation reaction product of aluminum oxide without any appreciable amount of the silicon metal reacting. Thus, the composition of the metal body after oxidation can differ, at least in terms of relative proportions, from the composition of the parent metal body before the oxidation reaction begins.

A solid, liquid or vapor phase oxidizer or a combination of such oxidizers may be used, as stated above. Typical oxidizers include, for example, without limitation, oxygen, nitrogen, a halogen, sulfur, phosphorus, arsenic, carbon, boron, selenium, tellurium, and compounds and combinations of these, e.g., silicon oxide (as an oxygen source), methane, ethane, propane, acetylene, ethylene and propylene (as carbon sources), and mixtures such as air, H₂/H₂O and CO/CO₂, the latter two (i.e., H₂/H₂O and CO/CO₂) being useful for reducing the oxygen activity of the environment.

A vapor phase (gas) oxidant is preferred and specific embodiments of the invention are described herein with reference to the use of vapor phase oxidants. When a gas or vapor phase oxidant is used, the filler bed or coating is permeable to the gas so that upon exposure of the filler coating to the oxidant, the vapor phase oxidant penetrates the filler bed and contacts the molten parent metal therein. The term "vapor phase oxidant" means a vaporized or normally gaseous material which creates an oxidizing atmosphere. For example, oxygen or oxygen-containing gas mixtures (including air) are preferred vapor phase oxidants, as in the case where aluminum is the parent metal, with air usually being more preferred for obvious economic reasons. When a vapor phase oxidizer is specified as containing or comprising a specific gas or vapor, it means an oxidizer in which the specified gas or vapor is the sole oxidizer of the parent metal under the conditions prevailing in the oxidizing environment used. For example, although the major constituent of air is nitrogen, the oxygen content of the air is the oxidizer of the parent metal because oxygen is a significantly stronger oxidizer than nitrogen. Air therefore falls within the definition of an "oxygen-containing gas" type oxidizer, but not within the definition of a "nitrogen-containing gas" type oxidizer. An example of a The "nitrogen-containing gas" type oxidizer as used herein and in the claims is "forming gas" containing approximately 96 volume percent nitrogen and 4 volume percent hydrogen.

When a solid oxidizer is employed, it is usually distributed throughout the entire embedment or filler coating, or in a portion of the coating adjacent to the parent metal, in the form of particles or powders mixed with the filler, or possibly as a film or coating on the filler particles. Any suitable solid oxidizer may be employed, including elements such as boron or carbon, or reducible compounds such as silicon dioxide or certain borides of lower thermodynamic stability than the boride reaction product of the parent metal. For example, when boron or a reducible boride is used as a solid oxidizer for an aluminum parent metal, the resulting oxidation reaction product is aluminum boride.

In some cases, the oxidation reaction with a solid oxidant may proceed so rapidly that the oxidation reaction product tends to fuse due to the exothermic nature of the process. This occurrence may reduce the uniformity of the microstructure of the ceramic body. This rapid exothermic reaction may be avoided by mixing relatively inert fillers of low reactivity into the assembly. Such fillers absorb the heat of reaction and minimize any possible thermal runaway effect. An example of such a suitable inert filler is one that is substantially the same as the desired oxidation reaction product.

When a liquid oxidizer is employed, the entire filler bed or a portion adjacent to the molten metal may be coated or soaked with the oxidizer to impregnate the filler. Reference to a liquid oxidizer means one that exists as a liquid under the conditions of the oxidation reaction, and so a liquid oxidizer may have a solid precursor, such as a salt, that is molten under the conditions of the oxidation reaction. Alternatively, the liquid oxidizer may be a liquid precursor or solution used to impregnate part or all of the filler and that is melted or decomposed under the conditions of the oxidation reaction to form a suitable oxidizing species. Examples of liquid oxidizers as defined herein include low melting glasses.

The filler material useful in the practice of certain embodiments of the invention is one which is permeable to the passage of the oxidant under the conditions of the oxidation reaction of the invention as described below when the oxidant is a vapor phase oxidant. In any event, the filler is also permeable to the growth or development of the oxidation reaction product therethrough. The filler also has sufficient cohesive stability at the processing temperature, which initially or rapidly forms or develops, so that it retains the external dimensions repeated in it by conformation of the filler to the body of base metal when molten metal is discharged from the cavity, with simultaneous formation of the cavity, the initially filled by the metal body. When a gaseous oxidant is used, the oxidation reaction product formed is generally impermeable to the surrounding atmosphere and therefore the furnace atmosphere, i.e. air, cannot penetrate into the developing cavity. In this way a region of low pressure develops in the cavity formed by the migration of the molten parent metal. The developing skin of oxidation reaction product is usually too weak at first to resist the pressure differential thus built up across it, together with the force of gravity, so that without further support it tends to collapse inwards, causing at least part of the evacuated region to be filled by the molten parent metal, thereby losing the shape of the cavity originally presented by the metal body. To avoid this collapse, or partial collapse, it is preferable to select a filler that partially sinters or otherwise sufficiently sets or bonds to the growing layer of oxidation reaction product at a temperature above the melting point of the parent metal and near (but below) the temperature of the oxidation reaction, to provide structural stability from the outside of the cavity such that the repeated external dimensions of the mold are maintained in the developing cavity, at least until the growing structure of oxidation reaction product has attained sufficient thickness to support itself against the developed pressure differential across the wall of the cavity.

A suitable self-bonding filler is one that, at the appropriate temperature, either sinters naturally or can be caused to sinter or bond by suitable additives or modifications to the surface of the filler. For example, a suitable filler for use with an aluminum base metal using air as an oxidant consists of alumina powder with an added binder of silica in the form of fine particles on the alumina powder. Such mixtures of materials partially sinter or bond at or below the conditions of the oxidation reaction under which the ceramic matrix is formed. Without the silica additive, the alumina particles require much higher temperatures to bond. Another suitable class of filler materials includes particles or fibers that, under the conditions of the oxidation reaction of the process, form a skin of reaction product on their surfaces that tends to bond the particles in the desired temperature range. An example of this class of fillers, when aluminum is used as the base metal and air as the oxidant, are fine particles of silicon carbide [e.g. 28 um (500 mesh) and finer] which form a skin of silicon dioxide which binds them together in the correct temperature range for the oxidation reaction of the aluminum.

It is not necessary that the entire mass or bed of filler consist of a sinterable or self-binding filler or that they contain a sintering or binding agent, although such an arrangement is within the scope of the invention. The self-binding filler and/or the binding or sintering agent may also be distributed only in the region of the filler bed which adjoins the body of base metal and surrounds it in a thickness sufficient to form, when sintered or otherwise bonded, an encapsulation of the developing cavity which is of sufficient thickness and mechanical stability to prevent collapse of the cavity until a sufficient thickness of the oxidation reaction product has been achieved. Thus, it is sufficient for a "support zone" of filler enveloping the mold to comprise a filler which is inherently sinterable or self-bonding within the appropriate temperature range, or which contains a sintering or binding agent which is effective within the appropriate temperature range. As used herein and in the claims, a "support zone" of filler is that thickness of filler enclosing the base metal body which, after setting, is at least sufficient to provide the structural stability required to maintain the repeated external dimensions of the metal body until the growing oxidation reaction product can support itself against collapse of the cavity, as described above. The size of the filler support zone will vary depending on the size and configuration of the metal body and the mechanical stability provided by the sinterable or self-bonding filler in the support zone. The support zone may extend from the surface of the metal body into the filler bed for a distance less than that over which the oxidation reaction product grows, or for the entire growth distance. In fact, in some cases the support zone may be quite thin. For example, although the filler support zone may be a filler bed encapsulating the base metal and itself enclosed by a larger bed of filler which is not self-bonding or non-sinterable, in appropriate cases the support zone may consist merely of a coating of self-bonding or sinterable particles attached to the mold with a suitable adhesive or coating.

In any event, the filler should not sinter, fuse or otherwise react to form an impermeable mass and block the infiltration of the oxidation reaction product therethrough or, if a vapor phase oxidant is used, the passage of that vapor phase oxidant therethrough. Furthermore, any sintered mass that forms should not form at temperatures so low that it fractures due to a mismatch in the expansion of the metal and the filler before the growth temperature is reached, thereby creating a non-homogeneous composite during the development of the matrix since the matrix would subsequently simply fill in the fractures in the bonded filler. For example, an aluminum base metal not only undergoes thermal expansion when the solid or molten metal is heated, but also undergoes a significant increase in volume when melted. This makes it necessary that the filler bed in which the base metal mold is embedded does not sinter or otherwise self-bond to form a solid structure that encloses the base metal mold before it expands in comparison to the filler, since otherwise the expansion would burst the self-bonded structure.

A binding or sintering agent may be present as a component of the filler in cases where the filler would otherwise not have sufficient self-binding or sintering characteristics to prevent collapse of the ceramic layer. formed into the volume previously occupied by the base metal. This binder may be distributed throughout the filler or in a region near or adjacent to the base metal. Suitable materials for this purpose include organometallic materials which, under the oxidizing conditions required to form the oxidation reaction product, at least partially decompose and bind the filler sufficiently to provide it with the required mechanical stability. The binder should not interfere with the process of the oxidation reaction or leave undesirable byproducts in the ceramic composite product. Binders suitable for this purpose are well known to those skilled in the art. For example, tetraethyl orthosilicate is a typical example of a suitable organometallic binder which, at the temperature of the oxidation reaction, leaves behind silicon oxide which effectively binds the filler with the required cohesive stability.

As explained in the patent applications of the same applicant, dopant materials used in conjunction with the parent metal beneficially affect the oxidation reaction process, particularly in systems using aluminum as the parent metal. The dopant or dopants used in conjunction with or in conjunction with the parent metal (1) may be available as alloying constituents of the parent metal, (2) may be applied to at least a portion of the surface of the parent metal, or (3) may be applied to or incorporated into all or a portion of the filler material, or any combination of two or more of the techniques (1), (2), or (3) may be used. For example, an alloyed dopant may be used alone or in conjunction with a second externally applied dopant. In the case of technique (3), where one or more additional dopants are applied to the filler material, the application may be carried out in any suitable manner, as explained in the patent applications of the same applicant.

The function or functions of a particular dopant material may depend on a number of factors. Such factors include, for example, the particular parent metal, the particular combination of dopants when two or more dopants are used, the use of an externally applied dopant in combination with a dopant alloyed into the precursor metal, the concentration of the dopant, the oxidizing environment and the process conditions.

Useful dopants of an aluminum base metal, particularly with air as the oxidant, include magnesium, zinc and silicon, either alone or in combination with other dopants as described below. These metals, or a suitable source of the metals, can each be alloyed into the aluminum-based base metal in concentrations between about 0.1-10 weight percent based on the total weight of the resulting doped metal. It should be noted, however, that certain dopants also form useful alloys with the base metal so that they have optimal heat storage characteristics and therefore the dopant can be alloyed in eutectic areas. These dopant materials or a suitable source thereof (e.g. MgO, ZnO or SiO2) may be used external to the parent metal. Thus, an alumina ceramic structure can be made from the aluminum-silicon parent metal using air as the oxidant when MgO is used as the dopant applied to the metal surface in an amount greater than about 0.0008 g per gram of parent metal to be oxidized and greater than 0.003 g per square centimeter of parent metal surface to which the MgO is applied.

Other examples of dopant materials for aluminum base metals include sodium, germanium, tin, lead, lithium, calcium, boron, phosphorus and yttrium, which may be used individually or in combination with one or more dopants, depending on the oxidant and process conditions. Rare earth elements such as cerium, lanthanum, praseodymium, neodymium and samarium are also useful dopants, and again especially when used in combination with other dopants. These dopants are effective in promoting the growth of the polycrystalline oxidation reaction product for aluminum-based base metal systems, as explained in the patent applications of the same applicant.

The following non-limiting example of the present invention is intended for illustrative purposes.

example 1

Three cylindrically shaped castings of an aluminum alloy designated 380.1 (from Belmont Metals, with a stated nominal weight composition of 8-8.5% Si, 2-3% Zn and 0.1% Mg as active dopants, and 3.5% Cu plus Fe, Mn and Ni, with the actual Mg content sometimes being higher, such as in the range of 0.17-0.18%) 2.2 cm (7/8 in) high and 2.54 cm (1 in) in diameter were machined to round their edges. A thin layer, approximately 2.5 mm (0.10 in) thick, of a coating material consisting of 50 weight percent alumina powder (C-75, unground, from Alcan Aluminium Ltd.), 20 weight percent alumina powder (C-71, uniformly ground, from Alcan Aluminium Ltd.), and 30 weight percent clay (from Edgar Plastic Kaolin) was evenly applied to the surface of each casting. The coating was allowed to dry and the coated castings were immersed in a barrier material consisting of a uniform mixture of 70 weight percent wollastonite fibers (a mineral calcium silicate, FP grade, from Nyco Inc.) and 30 weight percent gypsum (Bondex, from Bondex Inc.) contained in a refractory crucible. This assembly of the crucible and its contents was placed in a furnace charged with air as an oxidant and heated to 95ºC over 4 hours. The furnace was held at 950ºC for 60 hours and then cooled to ambient temperature over 4 hours.

The crucible and its contents were removed from the furnace and the three products were taken out of the crucible. Excessive barrier material was removed from their surfaces by light sandblasting. Examination of the products showed that the Oxidation reaction product had infiltrated the coating material. One of the three thermal energy storage media was cross-sectioned to show the metal substrate and void space. FIGURE 2 is a photograph showing the product after manufacture and FIGURE 1 is the cross-section of the product showing the ceramic container 4, metal body 6 and void space 8. The product was heated five times in succession from room temperature to 700ºC, beyond the melting point of the metal core, and cooled back to room temperature. No cracking or failure of the ceramic container was detected or observed.

Claims (14)

1. A method for producing a heat storage medium for direct contact, which has a body made of base metal and a coherent ceramic layer which is integrally formed with the metal body and encapsulates this base metal body, the method comprising: (a) heating a body of parent metal in the presence of an oxidizing agent to a temperature above the melting point of said parent metal but below the melting point of its oxidation reaction product formed in step (b) to form molten parent metal, and (b) at the temperature mentioned (i) reacting said molten parent metal with said oxidizing agent outwardly from the surface of said parent metal body to form a layer of oxidation reaction product integral with the parent metal body, (ii) transporting said molten parent metal through said oxidation reaction product into contact with said oxidizing agent so that fresh oxidation reaction product continues to form at the interface between said oxidizing agent and previously formed oxidation reaction product, thereby continuously forming an increasingly thicker layer of oxidation reaction product outwardly from said surface and simultaneously withdrawing molten metal from said body, (iii) continuing said reaction for a time sufficient to develop said progressively thicker layer to a thickness sufficient to substantially completely encapsulate unreacted base metal and to maintain a void resulting from said stripping, and (c) recovering the resulting heat storage medium. 2. A method for producing a heat storage medium comprising a body made of base metal and a coherent ceramic composite container which is formed integrally with and encapsulates said base metal body, the method comprising (a) providing a mass of filler material adjacent to the surface of a body of base metal, (b) heating said body of parent metal in the presence of an oxidizing agent to a temperature above the melting point of said parent metal but below the melting point of its oxidation reaction product formed in step (c) to produce molten parent metal, (c) and, at said temperature, (i) reacting said molten parent metal with said oxidizing agent outwardly from the surface of said body to produce a layer of oxidation reaction product on said body, (ii) transporting said molten base metal through said oxidation reaction product toward said oxidant and into said filler mass so that fresh oxidation reaction product continues to form at the interface between said oxidant and previously formed oxidation product, thereby continuously forming an increasingly thick layer of oxidation reaction product which infiltrates said filler material outwardly from said surface and simultaneously withdraws molten metal from said body, (iii) continuing said reaction for a time sufficient to develop said progressively thicker layer to a sufficient thickness to embed at least a portion of said filler material and thereby substantially encapsulate unreacted parent metal and to obtain a void resulting from said stripping, and (d) recovering the heat storage medium formed. 3. A method according to claim 1 or claim 2, further comprising providing a barrier element at least partially spaced from said body of parent metal, and continuing said reaction at least until said oxidation reaction product reaches said barrier element to produce said layer having a thickness defined by said barrier element. 4. The method of claim 3 wherein said base metal comprises aluminum, said oxidizing agent comprises an oxygen-containing gas, and further comprising using a dopant in association with said base metal, and wherein said barrier element is gas permeable and comprises a material selected from the group consisting of gypsum, Portland cement and calcium silicate and mixtures thereof. 5. The method of claim 2, wherein said filler material comprises at least one material selected from the group consisting of a metal oxide, a boride, a carbide and a nitride. 6. The method of claim 2 further comprising providing a source of a solid oxidizing agent and incorporating said source into said filler material and reacting said source with said parent metal to form said oxidation reaction product. 7. A method according to claim 1 or claim 2, wherein said base metal comprises an aluminum-based alloy having at least one alloying constituent selected from the group consisting of silicon and magnesium. 8. The method of claim 7, wherein said alloy comprises an alloy selected from the group consisting of eutectic alloys, hypereutectic alloys and ternary alloys. 9. The method of claim 1 or claim 2, wherein said parent metal comprises a material selected from the group consisting of silicon, titanium, zirconium, hafnium and tin. 10. The method of claim 2, wherein said filler material is applied to said base metal body as a coating. 11. The method of claim 10 further comprising disposing a barrier element adjacent to said coating material and opposite the surface of said base metal body. 12. A heat storage medium comprising a metal core derived from a base metal body and a coherent ceramic container integral with the said metal core and encapsulates it, said container comprising a three-dimensionally interconnected product of the oxidation reaction of a portion of said base metal body with an oxidizing agent at a temperature above the melting point of said base metal, while said body of base metal is reduced by said amount to form the core, and at least one cavity between said metal core and said ceramic container. 13. The heat storage medium of claim 12, further comprising a filler material substantially embedded by said ceramic container. 14. Use of a heat storage medium according to one of claims 12 or 13 or the product obtained by a process according to one of claims 1 to 11, as a heat storage medium for direct contact.